Conventional carpet products are well known wherein a facing layer of pile yarns if affixed to a suitable backing fabric of polypropylene or the like. One of the most common problems associated with such carpet products concerns the ability of the carpet to lie flat on a floor surface under conditions of normal use. In addition to being subjected to concentrated stresses applied to the surface of the carpet, such as pedestrian traffic or an object being moved or wheeled across the carpet, a carpet may undergo changes in dimension resulting from variations in the environment. Such dimensional changes can result from exposure to thermal extremes, or by absorption or loss of moisture resulting from fluctuations in humidity. These changes in dimension may cause the carpet to wrinkle or buckle when it swells or expands, or may cause carpet seams to separate when contraction or shrinkage occurs.
In an effort to stabilize carpet products against concentrated stresses and against variations in the environment, it is known to incorporate into the carpet an additional fibrous layer that is resistant to stresses and is minimally affected by temperature and humidity changes. Woven or nonwoven fiberglass fabrics are notably useful for this purpose and effectively inhibit deformation of the carpet under stress, as well as dimensional changes in the carpet caused by environmental changes.
When glass fibers are used in a carpet, due to their inherent brittleness, they must be protected from breakage and prevented from migrating into the carpet pile. Otherwise, severe irritation to the user may result. To this end, it is known to encapsulate the fiberglass layer and adhere it to the carpet by means of a solid or cellular resilient backing material. Thus, a dimensionally stable carpet may be made from a layer of pile yarns positioned and supported by a suitable backing fabric, with the pile and backing fabric adhered to an underlying layer of encapsulated glass fibers.
Prior efforts have been made to overcome the deformation of carpets and carpet products resulting from concentrated stresses applied to the carpet's upper surface. Examples are found in U.S. Pat. No. 4,010,301 and its related U.S. Pat. No. 4,010,302, both of which disclose carpet products comprising a pile yarn facing layer adhered to an upper fiberglass stiffening and stabilizing layer. A layer of resilient thermoplastic material is disposed beneath the upper stiffening and stabilizing layer, and a second lower stiffening and stabilizing fiberglass layer is disposed beneath the thermoplastic layer. By controlling the thickness of the thermoplastic layer between the two fiberglass stiffening and stabilizing layers, the bend axis of the composite carpet tile is located closer to the upper stiffening and stabilizing layer than to the lower stiffening and stabilizing layer. Due to this larger moment arm below the bend axis of the carpet, the application of a lateral force to the top surface of the carpet, imparts to the carpet a greater tendency to bend downward than to bend upward. This improves the free-lay nature of the carpet, since the tendency of the carpet to hug the floor overrides the tendency of the carpet to turn upward under concentrated loads.
While the carpet products of the aforementioned U.S. Pat. Nos. 4,010,301 and 4,010,302 achieve some degree of success in resisting concentrated stresses exerted upon the surface of the carpet, they fail to address the problems caused by stresses created within the carpet resulting from exposure to temperature extremes or to changes in humidity. One such problem, commonly encountered during the manufacturing process, concerns the tendency of the polypropylene fabric commonly used as a primary backing layer to anneal when exposed to temperatures higher than 175.degree. F. Exposure to such high temperatures can cause permanent shrinkage of the polypropylene. In contrast, the fiberglass layer is virtually impervious to the temperatures encountered during the manufacturing process. Thus, the different layers of the carpet are affected differently by the high temperatures of the manufacturing process. This difference in the response of the various layers to temperature extremes causes stress in the backing fabric layer as it attempts to change dimensions but is restricted from doing so by the more stable fiberglass layer. This stress can be relieved only by a bending of the carpet similar to the bimetallic bending commonly seen in thermostats and the like. This bending can cause portions of the carpet to lift upwardly off the floor, as the carpet tends to assume a concave or "curled" orientation. Efforts to overcome this stress problem by preshrinking the polypropylene layer, annealing it prior to attaching it to the more stable fiberglass layer, have been largely unsuccessful due to wrinkles and distortions in the annealed polypropylene layer and the difficulty in handling and in attaching the distorted preshrunk layer to the fiberglass layer.
A further problem is found when a conventional carpet product is exposed to changes in the ambient humidity. The various materials commonly used in carpet construction exhibit a wide disparity in water absorption characteristics. For example, wool and cellulosics such as cotton, jute, ramie, and sisal are capable of absorbing up to 15% of their weight in moisture. Nylon can absorb 6-8% of its weight in moisture, while polypropylene absorbs less than 0.1% of its weight in moisture and fiberglass virtually none. Thus, the different materials comprising carpet react differently to changes in humidity, as the different components absorb varying amounts of moisture. The more moisture a material absorbs, the more the material will tend to expand; and conversely, the more moisture a material loses, the more it will tend to shrink.
These variations in the hydrophilicity of the different materials comprising the carpet cause stresses within the carpet as it is exposed to changes in the ambient humidity. As the pile yarns absorb moisture, they tend to expand. However, since the adhesive anchoring the pile yarns to the backing fabric fills the interstices between adjacent pile yarn loop backs, such absorption of moisture causes stress in the pile yarn and backing fabric as they attempt to expand but are restricted from doing so by the more stable fiberglass layer beneath. Conversely, loss of moisture from the pile yarns can also cause stress as the pile yarn and backing fabric layer attempt to shrink but are restricted from doing so by the fiberglass layer. These stresses can again be relieved only by a bending of the carpet: a convex bending or "doming" in the case of moisture absorption, and a concave bending or "curling" in the case of moisture loss. Again, the bending can cause portions of the carpet to lift up off the floor, causing the carpet to become unsightly and hazardous. Unlike the problems presented by temperature extremes, which typically occur only during the manufacturing process, the problems presented by variations in the ambient humidity can plague a carpet product even after installation.